Method and device for controlling stirring in a strand

ABSTRACT

A method of controlling the flow of molten metal in non-solidified portions of a strand ( 3 ) during continuous casting, and a device for application of the method. The device comprises a two-phase or plural-phase stirrer ( 1 ) arranged around a mould ( 6 ) that is open in opposite ends and that surrounds the strand ( 3 ), said stirrer ( 1 ) being arranged to generate a moving magnetic field in the melt, and at least one magnetic flow-conducting body ( 15 ) being placed between an inner periphery of said stirrer ( 1 ) and the outer periphery of the mould ( 6 ). In the case of open casting, the magnetic flow-conducting body ( 15 ) is positioned in a first position in which it displaces at least a part of the magnetic flow generated by the stirrer ( 1 ) in a direction towards the meniscus ( 17 ) of the strand ( 3 ), and, in the case of closed casting, the magnetic flow-conducting body ( 15 ) is positioned in a second position in which it displaces at least a part of the magnetic flow generated by the stirrer ( 1 ) away from the meniscus ( 17 ) of the strand ( 3 ).

TECHNICAL FIELD

The present invention relates to a method for controlling the stirringof melted metal in non-solidified portions of a strand during acontinuous casting, by which the melt is affected by means of atwo-phase or plural-phase stirrer arranged around a mould that is openin opposite ends and that surrounds the strand, and where at least onemagnetic flow-conducting body is placed between an inner periphery ofsaid stirrer and the outer periphery of the mould.

The invention also relates to a device for controlling the stirring ofmelted metal in non-solidified portions of a strand during continuouscasting, comprising a two-phase or plural-phase stirrer arranged arounda mould that is open in opposite ends and that surrounds the strand, thestirrer being arranged to generate a moving magnetic field in the melt,and at least one magnetic flux-conducting body being placed between aninner periphery of said stirrer and the outer periphery of the mould.

The term stirrer should be regarded in a wide meaning, but is primarilyreferred to as electro-magnetic stirrers that comprise one or moremagnet cores around which windings, coils, are arranged for the purposeof generating a magnetic field in a melt located between these coreswhen the stirrer is subjected to an electric current. In the case ofonly one core, this core can be arranged as a toroid around the mould,while in the case of a plurality of cores, the latter can be connectedvia a common yoke made of a suitable material, for example iron.

Mould is referred to as all types of open moulds that are used inconnection to continuous casting of metals, but particularly those thatare used in connection with casting of slabs, blooms and billets, wherethe use of a stirrer is made in order to, amongst others, prohibit thegrowth of dendrites in a direction towards the centre of the strand asthe melt in the latter solidifies.

THE BACKGROUND OF THE INVENTION

When a metal or metal alloy such as steel is cast in a mould that isopen in both ends in the casting direction by means of a continuous orsemi-continuous process, a melt is delivered to the mould as a freetapping jet (open casting) or via a casting tube (closed casting). Asthe melt passes through the mould a strand is formed by the cooled melt.Before the strand leaves the mould at least a self-supporting surfacelayer has been formed on the strand. An uncontrolled flow of metal inthe non-solidified portions of the strand results in problems withregard to quality as well as the production itself.

The flow of melted metal in the non-solidified portions of a strand canbe controlled by means of a two-phase or plural-phase stirrer by meansof which a rotating magnetic field can be forced to act on the melt inthese portions of the strand, as electric currents are induced in themelt. Thereby, due to a common effect of the magnetic field and thecurrents, stirring forces are created in the melt. A typical stirrer canbe compared to the stator of an electric motor, where the melt forms theinner rotor. However, the losses are larger than by a conventionalmotor, partly depending on the gap that, for the majority of the stirrerconstructions, is present between the cores and the melt.

In order to obtain the required string force, and accordingly satisfyingmetallurgical results, the movements of the melt must be controlled anddistributed. Therefore, there are requirements on the properties and thedistribution of the field strength of the magnetic field that is appliedto the melt, as well as on the current intensity and the current densityof the induced currents.

The requirements may differ between different casting processes, such asopen and closed casting.

By closed casting, that is, when the melt is delivered to the mouldthrough a casting tube that outlets in the melt below the so-calledmeniscus, there is required a stirring in the non-solidified portions,said sting being of enough strength to guarantee the requestedmetallurgical result as to the casting structure. However, stirring inthe meniscus area should be avoided because such stirring could resultin slag and casting powder on the meniscus surface being pulled downinto the melt and then generating inclusions therein as the meltsolidifies.

By open casting, that is, when melt is delivered to the mould from acontainer or a box or the like, as a free jet, it is required that theflow velocity at the meniscus is sufficiently high in order to obtainthe requested metallurgical results. A sufficiently strong flow at themeniscus is obtained by giving the applied magnetic flew-flux asufficiently high strength at the meniscus.

PRIOR ART

In order to achieve the requirements mentioned above with regard to themagnetic flew flux at the meniscus during open as well as closedcasting, the magnetic field should be moved from a first position closeto the meniscus during open casting to a position further away, belowthe meniscus during closed casting. The document DE 38 19 493 describesa device for continuous casting by which this problem has been solved byhaving the stirrer, with existing coils and cores, movably arranged inthe casting direction along an inner mould. However, in reality, such asolution is complicated and will require a large space for the stirrer.DE 28 53 049 discloses how the magnetic flew-flux from a linear motorstirrer is amplified as magnetic teeth are arranged between the stirrercoils and the strand. The teeth are removably arranged and, for example,made of transformer sheet. However, this document does not mention howthe field can be directed upwards or downwards such that the stirrer canbe located at the same level for both open and closed casting.

THE OBJECTION OF THE INVENTION

The object of the present invention is to propose a method and a devicefor controlling the stirring of melted metal in non-solidified parts ofa strand during continuous casting by means of a rotating magneticfield, said magnetic field being displaceable in a direction towards orfrom the meniscus of the strand depending on if an open or closedcasting is applied. At the same time, the invention shall permit theenergy losses of the stirrer to be kept at a minimum, as an existing gapwith poor magnetic flow-conducting properties between the coils of thestirrer and the melt is minimised.

SUMMARY OF THE INVENTION

The object of the invention is attained by means of the initiallydefined method, characterised in that, in the case of open casting, themagnetic flux-conducting body is positioned in order to displace atleast a part of the magnetic flux generated by the stirrer in adirection towards the meniscus of the strand, and that, in the case ofclosed casting, the magnetic flew flux-conducting body is positioned inorder to displace at least a part of the magnetic flux generated by thestirrer away from the meniscus of strand. In normal cases, the mould isarranged generally vertically. Therefore, the magnetic flux-conductingbody is positioned closer to the top of the mould during open castingand further downwards, away from the top, during closed casting.

It should be realised that a plurality of magnetic flew flux-conductingbodies can be positioned around the mould in the way mentioned above.Preferably, a respective body is positioned generally opposite to, thatis, radially inside, a respective pole in order to define an extensionof said pole in a direction towards the mould. However, there is nothingthat prohibits said body from extending axially in the casting directionbeyond the corresponding pole of the stirrer, that is, that the body isat least partly on a higher or lower level than the cores and coils ofthe stirrer. In that way, the magnetic field is directed towards aregion in which it would only have been present to a small degree inabsence of the supplementing magnetic flux-conducting body or bodies.

A preferred embodiment of the method is characterised in that themagnetic flux-conducting body is arranged to conduct a major part of themagnetic flux generated by the stirrer. Thereby, it is preferred thatthe magnetic flux-conducting body is dimensioned in such a way and madeof such a material that generally the whole magnetic field generated bythe stirrer is conducted into the melt via said body withoutsuper-saturation thereof.

In the case in which the mould is surrounded by an outer mantle which,between itself and the outer periphery of the mould defines one or morecooling channels, the magnetic flew flux-conducting body is preferablypositioned between the outer periphery of the mould and this outermantle. In the casting of billets, blooms and slabs the present trend isto use moulds where the copper mould is surrounded by such an outermantle and where the mould and the mantle together form a cassette whichcan be brought into and out of the surrounding stirrer in one piece, forexample when new dimensions are to be cast or by a change from closed toopen casting. The invention includes solutions in which the magneticflux-conducting bodies are positioned at different levels in differentcassettes, with regard to the application of open or closed casting, orwhere the body/bodies is/are movably arranged in a cassette in order tobe able to positioned depending on the casting case in question.Thereby, the method according to the invention promotes an effectiveutilisation of existing space between mould and stirrer while,simultaneously, the advantages of cassette solutions are permitted.

The object of the invention is also achieved with a device of theinitially defined type, characterised in that, in the case of opencasting, the magnetic flux-conducting body is positioned in a firstposition in which it displaces at least a part of the magnetic fluxgenerated by the stirrer in a direction towards the meniscus of thestrand, and that, in the case of closed casting, the magneticflux-conducting body is positioned in a second position in which itdisplaces at least a part of the magnetic flux-generated by the stirrerway from the meniscus of the strand.

Preferably, the magnetic flux_conducting body is arranged to conductmajor part of the magnetic flux generated by the stirrer to the melt. Ashas been previously mentioned, the magnetic flew-flux-conducting body ispreferably dimensioned in such a way and made of such a material thatgenerally the whole magnetic field generated by the stirrer is conductedinto the melt via said body without any super-saturation thereof, or inany of a plurality of such bodies. One way of achieving this is to makeuse of a plurality of rod-formed or plate-formed magnetic fieldflux-conducting bodies of a suitable material and of a suitable size,aid bodies being positioned between the respective poles of the stirrerand the mould. Preferably, the number of bodies corresponds to thenumber of poles of the stirrer, the number thereby being twice thenumber of phases as there is needed two poles for each phase in aplural-phase stirrer. The bodies may, preferably, be elongated andextend in the casting direction.

According to one embodiment, the mould is surrounded by an outer mantlewhich between itself and the outer surface of the mould defines one morecooling channels, the magnetic flux-conducting body being positionedbetween the outer periphery of the mould and the outer mantle. A coolingmedium, preferably water, fills the space between the mantle and themould. The magnetic flux conducting body (or bodies) is (are) preferablymainly made of iron, but may, advantageously, contain corrosionstrength-increasing alloy elements or be provided with a layer or anycorrosion resistant material in order to prevent the from corroding dueto the effect of the cooling medium.

In reality, the mould is often surrounded by a supporting sheet, and themagnetic flew flux-conducting body is then, according to one embodiment,fixed to the support sheet. According to an alternative embodiment, themagnetic flux-conducting body or bodies) is (are) fixed to the outermantle, more precisely the inner periphery thereof.

Further advantages and features of the invention will be disclosed inthe following detailed description and in the enclosed dependent patentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, the invention will be described more in detail withreference to the embodiments that are shown on the annexed drawings, onwhich:

FIG. 1 is a schematic cross-section from one side of a first embodimentof the device according to the invention, with magnetic flux-conductingbodies in a first position.

FIG. 2 is a cross-section corresponding to the one in FIG. 1, but withthe magnetic flux-conducting bodies in a second position.

FIG. 3 shows a cross-section from one side that shows a device accordingto a first embodiment, corresponding to the one in FIGS. 1 and 2,

FIG. 4 is a cross-section according to IV—IV in FIG. 3,

FIG. 5 is a cross-section corresponding to the one in FIG. 3, butshowing a second embodiment of the device according to the invention,

FIG. 6 is a cross-section according to VI—VI in FIG. 5,

FIG. 7 is a cross-section corresponding to the one in FIGS. 3 and 5, butshowing a third embodiment of the device according to the invention,

FIG. 8 is a cross-section according to VIII—VIII in FIG. 7,

FIG. 9 is a cross-section corresponding to the ones in FIGS. 3, 5 and 7,but showing a fourth embodiment of the device according to theinvention, and

FIG. 10 is a cross-section according to X—X in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a first embodiment of a device according to theinvention, intended for continuous casting of metals, particularlysteel. FIG. 1 shows the device in a first position, adjusted for opencasting, that is, casting with a free tapping jet, while FIG. 2 showsthe device in a second position, adjusted for closed casting, that is,casting by means of a casting tube.

The device comprises an electromagnetic stirrer 1 that comprises aplurality of cores 2, as shown in FIG. 4. Coils 4 of electric conductorsare wound around the cores. In the shown embodiment the device comprisesthree pairs of poles in a way known per se, and these poles are fed witha three-phase alternating current in a way known per se. Alternativesolutions may, for example, comprise two pairs of poles which are fedwith two-phase alternating current. The cores 2 are interconnected by ayoke 5. Preferably, the cores and the yoke are substantially made ofiron. The alternating current frequency is low, for example in the rangeof 1-20 Hz.

As an alternative, the device may comprise only one iron core, arrangedas a toroid that extends around the mould and on which a correspondingnumber of coils are wound in a way known per se.

The stirrer 1 is arranged around a mould tube or a mould 5 that suitablycomprises copper as its major constituent, said mould being arranged forgenerally vertical casting and presenting an upper opening 7, throughwhich melt is delivered to the mould, and a lower opening 8, throughwhich a strand 3 is continuously taken out from the mould 6 duringcasting.

In the embodiments shown in FIGS. 1-6 the device comprises a cassettethat comprises the copper mould and an outer mantle 9 that is arrangedoutside and with a distance to the mould. A space 10 between the mould 6and the mantle 9 defines a cooling channel. FIG. 5 shows that the mantle9 comprises an inlet hole 11, via which a cooling medium, preferablywater, is introduced inside the mantle 9 in a separate lower space 12 inorder to subsequently being brought to flow along the outer periphery ofthe mould 6 upwards and thereby cooling the latter. A support sheet 13,suitably made of stainless steel, is arranged most adjacent outside themould 6, and the cooling medium is flowing from the lower space in a gapbetween the mould 6 and the support sheet 13 upwards as seen in thefigure. At the upper end of the support sheet 13 the gap opens in thespace 10, and flows through the latter downwards and out of said changervia an outlet opening 14 in the lower part of the mantle 9, just above atransverse wall 16 that separates the space 10 from the lower space 12.It should be realised that, of course, a plurality of alternatives tothis solution are within the scope of the invention and that thissolution is only shown by way of example. For example, it is fullypossible to arrange an inlet, an outlet and a transverse wall thatcorrespond to the inlet 11, the outlet 14 and the transverse wall 16, inthe upper part of the mould, which, in reality, also is often the case.The mantle 9 and the support sheet 13 must, however, be made ofnon-magnetic material.

Between the stirrer 1 and the mould 6 there is arranged a plurality ofbodies 15 of a magnetic flux-conducting material, preferably iron. Thenumber of magnetic flux-conducting bodies corresponds to the number ofpoles of the stirrer 1, in this case six. Each of the bodies 15 isarranged adjacent a respective pole, radially inside the latter, inorder to improve the magnetic conductivity between the pole in questionand the strand 3 located inside the mould. In FIG. 1, the bodies 15 arepositioned in an upper position in which they direct the magnetic fieldfrom the stirrer upwards towards the region of the meniscus 17 of thestrand. Thereby, the bodies 15 are displaced towards or even partlybeyond an upper part or upper half of the axial region over which thepoles of the stirrer 1 extend. Thereby, it will be possible to directthe magnetic field generated by the stirrer 1 towards the meniscus areaalso in the case when the meniscus is located at a higher level than thestirrer, thanks to a displacement of the position of the bodies 15upwards towards the meniscus 17.

FIG. 2 shows the case in which the bodies 15 are displaced towards oreven partly beyond a lower part or a lower half of the region over whichthe poles of the stirrer 1 extend. Thereby, the magnetic field generatedby the stirrer 1 is directed away from, downwards from the meniscusarea. Preferably, the bodies 15 are positioned in this second positionduring closed casting, when it is required to avoid a strong magneticfield at the meniscus 17.

FIGS. 3 and 4 show an embodiment in which the bodies are fastened to theinside of the outer mantle 9, and thereby placed in the space 10 betweenthe mantle 9 and the mould 6. Thereby, a part of the cooling channelspace is used in order to make place for the bodies 15, and this isadvantageous from a space-serving point of view. Thereby, the bodies 15form a part of the above-mentioned cassette.

FIGS. 5 and 6 show an alternative embodiment of a cassette by which thebodies 15 are fastened to a support sheet 13 that surrounds the coppermould 6. However, the principle is the same, namely to arrange thebodies 15 in the cooling channel space between the stirrer 1 and themould 6 in order to take advantage of the space that in any case will beprovided by the cassette, in order to reduce field strength losses and,at the same time, direct the magnetic field.

FIGS. 1-6 show examples of mould housings with externally locatedstirrers 1, that is, a cassette that comprises the mould 6 and the outermantle 9 which is separately removable from the rest of the stirrer 1.In this case, the removal is performed by lifting the cassettevertically upwards out of the stirrer 1.

FIGS. 7-10 show examples of a mould housing with internally locatedstirrers, that is, that it is not possible to distinguish anywell-defined cassette that is removable from the stirrer. Instead,stirrer and mould form one single unit. In such a case, when a cassettewith a given positioning of the bodies 15 cannot simply be substitutedto the cassette with another positioning of the bodies 15, for examplein connection with the transfer from open to closed casting, theimportance of having the bodies 15 displaceable or movable between theabove-mentioned first and second positions increases. This can beobtained as the bodies are displaceably arranged in the castingdirection along the outside of the mould 6 or, as shown in FIGS. 7 and8, the outside of the outer mantle 18. Alternatively, they aredisplaceably arranged along the inside of an outer mantle 9, such asshown in FIGS. 9 and 10. The mantles 18, 19 correspond to theabove-described mantle 9 as they are arranged in contemporaryconventional mould housings with included stirrers.

With regard to the above disclosure of the invention it should berealised that a plurality of further, alternative embodiments of coursewill be obvious to a man skilled in the art, however without goingbeyond the scope of the invention as the latter is defined in theenclosed patent claims supported by the description and the annexeddrawings.

For example, within the scope of the invention, it is also possible topresent alternative embodiments of the magnetic flux-conducting bodies15, and to divide separate bodies into a plurality of sub-bodies, etc.As one or more bodies 15 are divided into sub-bodies or segments one ormore of those sub-bodies should be removably arranged in order to enablea redirection of the magnetic field during ongoing casting or betweendifferent casting processes.

The above examples have shown stirrers with three-phase, two-polecoupling, resulting in the application of six bodies or pole plates 15.However, it should be realised that also other constructions arepossible. For example, stirrers with two-phase, two-pole couplings areoften used, resulting in the use of four bodies or pole plates, one ateach pole.

It is also conceivable that the poles of the stirrers are arranged atdifferent levels along the mould, and that this, in its turn, results inthe possibility of arranging the bodies/pole plates 15 at mutuallydifferent levels.

The bodies 15 are preferably laminated, that is, constructed as laminatestructures. However, they may be made of homogenous iron, but will thenoperate less well.

1. A method for controlling a stirring of molten metal in non-solidifiedportions of a strand during continuous casting, comprising the steps of:providing a two-phase or plural-phase stirrer arranged around a mouldthat is open at opposite ends and that surrounds the strand, and placingat least one magnetic flux-conducting body between an inner periphery ofsaid stirrer and the outer periphery of said mould, positioning, in caseof open casting, the magnetic flux-conducting body in order to displaceat least a part of the magnetic flux generated by the stirrer in adirect on towards the meniscus of the strand, and positioning, in thecase of closed casting, the magnetic flux-conducting body in order todisplace at least a part of the magnetic flux generated by the stirrer aay from the meniscus of the strand.
 2. A method according to claim 1,further comprising the step of arranging the magnetic flux-conductingbody to conduct a major part of the magnet flew flux generated by thestirrer.
 3. A method according to claim 1, further comprising the stepsof: dimensioning the magnetic flux-conducting body in such a way andforming it of such a material that generally the whole magnetic fieldgenerated by the stirrer is conducted into the melt via said bodywithout any supersaturation of said body.
 4. A method according to claim1, comprising the steps of: positioning the mould vertically with anupper and lower opening respectively, and displacing the magnetic fieldupwards along the mould in the case of open casting and downwards in thecase of closed casting.
 5. A method according to claim 1, comprising hesteps of: surrounding the mould by an outer mantle that, between itselfand the outer periphery of the mould, defines one or more coolingchannels, and positioning the magnetic flow-conducting body between theouter periphery of the mould and the outer mantle.
 6. A method accordingto claim 1, comprising the steps of: surrounding the mould by a supportsheet and fastening the magnetic flux-conducting body to the supportsheet.
 7. A method according to claim 5, comprising the step of:fastening the magnetic flux-conducting body to the outer mantle.
 8. Adevice for controlling the stirring of molten metal in non-solidifiedportions of a strand during continuous casting, comprising: a two-phaseor plural-phase stirrer arranged around a mould said stirrer being openin opposite ends and surrounding the strand, said stirrer being arrangedto generate a moving magnetic field in the melt, and at least onemagnetic flux-conducting body, located between an inner periphery ofsaid stirrer and the outer periphery of the mould, wherein in the caseof open casting, the magnetic flux-conducting body is in a firstposition proximate to the menicus such that it displaces at least a partof the magnetic flux generated by the stirrer in a direction towards themeniscus of the strand, and, in the case of closed casting, the magneticflux-conducting body is positioned in a second position proximate to themelt and remote from the meniscus in which it is displaced at least apart of the magnetic flux generated by the stirrer away from themeniscus of the strand.
 9. A device according to claim 8, wherein whenthe magnetic flux-conducting body is arranged to conduct positionedproximate to the melt major part of the magnetic flux generated by thestirrer is conducted by the body to the melt.
 10. A device according toclaim 8, wherein the magnetic flux-conducting body is dimensioned insuch a way and is formed of a material the generally, the whole magneticfield generated by the stirrer is conducted into the melt via said bodywithout any supersaturation of said body.
 11. A device according toclaim 8, wherein the mould is surrounded by an outer mantle which,between itself and the outer periphery of the mould, defines one or morecooling channels, and the magnetic flux-conducting body is positionedbetween the outer periphery of the mould and the outer mantle.
 12. Adevice according to claim 8, wherein the mould is surrounded by asupport sheet and the magnetic flux-conducting body is fastened to thesupport sheet.
 13. A device according to claim 11, wherein the magneticflux-conducting body is fastened to the outer mantle.
 14. A deviceaccording to claim 8, wherein the mould is arranged generally verticallywith an upper and a lower opening for continuous casting in a verticaldirection, and the magnetic flux-conducting body is arranged at an upperregion of the mould adjacent the meniscus during open casting, and at alower region of the mould remote from the meniscus during closedcasting.
 15. A device according to claim 8, further comprising one ormore pole pairs arranged around the mould, and the at least one magneticfield flux-conducting body is arranged between at least one of the polesof the pole pair and the mould.
 16. A device according to claim 8,further comprising one or more pole pairs arranged around the mould, andthe at least one magnetic field flux-conducting body is arranged betweenthe pair of poles and the mould.
 17. A device according to claim 8,wherein the least one magnetic flux-conducting body has the shape of arod or a plate that extends in the longitudinal direction of the mould.18. A device according to claim 8, wherein the stirrer and the mould arein spaced relation, the at least one magnetic flux-conducting body beinglocated in the space and being sized in order to permit movement of theat least one magnetic flux-conducting body from the first to the secondposition and from the second to the first position in the space withoutremoving of the mould from the stirrer.
 19. A device according to claim18, wherein the a least one magnetic flux-conducting body isdisplaceably arranged between the first and the second position.